The present invention relates generally to a seat assembly structure for an automotive vehicle, which seat assembly includes an adjustable thigh support. More specifically, the invention relates to a thigh support adjusting mechanism of an automotive vehicle seat which requires a relatively small adjusting force for adjustment of the tilting angle of the thigh support for selecting the best seat position.
Various constructions of vehicle seats have been developed and are available in the market. Recently vehicle seats, at least the driver's seat, are being made adjustable not only for the position of the seat with respect to steering wheel but also the seat hight, seat back support and seat cushion tilt angle. A thigh support is provided for such adjustable seats in order to provide the best foot position with respect to the vehicle pedals. Some such adjustable seats have thigh support adjusting mechanisms which are independent of the remaining seat cushion parts. For example, Japanese Patent Application No. 53-143833 which is published under Tokkai. No. Sho. 55-70214 on May 27, 1980, illustrates a thigh support structure capable of adjustment of the tilting angle independently of the remaining part of the seat cushion. The thigh support structure of Tokkai. No. Sho. 55-70214 has an adjusting mechanism including an operation lever which is manually operated and a pushing lever positioned below the thigh support section separated from remaining sections of the seat cushion so that it can be moved independently of the latter. The rotational operating force applied to the operation lever is transmitted to the pushing lever via a rotatable rod.
In such a conventional construction, the rotational force applied to the operation lever is directly transmitted to the pushing lever, and consequently, the required adjustment force to be applied to the operation lever is relatively large. This will be more clearly understood from the following example. FIG. 1 illustrates a pushing lever A which is connected to the operation lever (not shown) via the rotatable rod R which has a longitudinal axis P serving as a rotational axis of the pushing lever A. The pushing lever A is secured to the rotatable rod R so that it may rotate with the latter in response to a rotating force applied to the operation lever.
Assume that the effective length L' of the pushing lever A is 129 mm, the tilting angle .theta..sub.1 of the pushing lever A with respect to the horizontal plane is 25 degrees and the vertically applied downward force F due to weight of a passenger is 30 kg. The component F' of the force F applied perpendicular to the longitudinal axis of the pushing lever A and operating against the clockwise rotation of the pushing lever at the point R.sub.1 can be obtained from EQU F'=F.multidot.cos .theta..sub.1
Therefore, the rotational moment M in a direction opposite to the operational rotation moment is applied in accordance with the equation EQU M=F.multidot.cos .theta..sub.1 .multidot.L'=3507 kg.multidot.mm
As apparent from the foregoing, the adjustment force required for adjusting the tilting angle of the thigh support is quite large.